The aim was to explore the effect of steroid hormone on mammary immune function and the possibility that microbiota components can transfer to mammary gland. Estrogen caused the reduction of milk yield. When the microorganism components was infused into mammary gland, innate immune factors concentration in milk was high in the estrogen-injected animals compared to that in progesterone-injected animals. Intravenous injection of microorganism component induce inflammation of mammary gland. These results strongly suggest that estrogen reduces milk yield to elevate the concentration of innate immune factors, resulting in the establishment of strong defense system in mammary gland. It is also demonstrated that microbiota components can transfer to the mammary gland, which causes the mastitis. These findings contain noble and new knowledge and insights and can contribute to the protection and treatment of mastitis in the future.results strongly suggest that